This research addresses the fundamental question of what determines behavioral variation. Many environmental variables can produce predictable effects on phenotype. One such variable is the incubation temperature of eggs in many reptiles. In the leopard gecko, embryos become male or female depending upon their temperature during development. In addition, between-sex as well as within-sex differences attributable to incubation temperature have been found in morphology, secretion of and sensitivity to steroid hormones, sociosexual behavior, reproductive success, and in the neuroanatomy and metabolic activity of brain areas that mediate sociosexual behaviors. These environmental effects are analogous to the effect of intrauterine environment in mammals, including humans. Given the homology of the endocrine and nervous systems across vertebrates, it is important to determine if homologous mechanisms underlie these analogous effects on behavior. If the mechanism underlying environmental effects on behavior in the leopard gecko is conserved (i.e., via sex steroids), this research will lend new insight into the evolution of sexual differentiation because temperature-dependent sex determination is thought to be the evolutionary precursor to genotypic sex determination (present in birds and mammals) and because reptiles are the ancestors of both birds and mammals. If the mechanism is different (i.e., direct temperature effects), this research would elucidate a novel process of sexual differentiation that may also be present in birds and mammals but, because of homeothermy, is masked. This latter possibility is especially important because young birds and mammals cannot regulate their body temperature as do adults. The proposed research is broadly categorized into three groups: thermoregulation and its relation to sociosexual behaviors (i.e., the degree to which the neural substrates mediating thermoregulatory and sociosexual behavior overlap), the development of the neural phenotypes of these brain regions and hormone milieus, and the effects of hormonal manipulations during development and neural manipulations in adulthood on thermoregulatory and sociosexual behaviors. The final category of experiments is particularly important as it will discern whether the incubation temperature effects are direct or indirect.